Drilling of a U-mineralized breccia pipe near Blue Mountain, Hualapai Indian Reseveration, northern Arizona

Abstract

The Blue Mountain pipe is a solution-collapse breccia pipe located on the Hualapai Indian Reservation of northwestern Arizona, about 7 miles north of Blue Mountain and 4 miles southeast of Diamond Creek. The pipe formed intially by collapse of strata into large dissolution caverns within the underlying Mississippian Redwall Limestone. The caverns formed during the Late Mississippian; dissolution may have been continuous from the Mississippian into the Triassic or reactivated during the Triassic. Like all northern Arizona breccia pipes, strata collapsed downward into dissolution voids-no clasts have been found above their normal stratigraphic position. Continued collapse from the ceiling and steep walls of the cavern resulted in upward propagation of the pipe; the free fall of rock caused extensive brecciation. Distinguishable clasts from within the Blue Mountain pipe indicate a minimum downward displacement of at least 290 ft. The Blue Mountain pipe is exposed along a cliff of Coconino Sandstone on the western edge of the Coconino Plateau. At the surface the pipe contains a prominent silicified breccia column about 130 ft high, 200 ft wide, and 300 ft long. The pinnacle consists of highly silicified, angular fragments and blocks, primarily derived from the Coconino Sandstone, with some fragments recognizable as belonging to the overlying Toroweap Formation. Evidence that the silicified pinnacle is part of a uranium-mineralized breccia pipe, includes: 1) polylithologic, chaotically-oriented clasts, 2) brecciated quartz veins with quartz crystal-lined vugs exhibiting radioactivity up to 1,200 counts per second (40 times background), 3) extensive Liesegang banding of iron minerals, indicating secondary fluid movement, and 4) evidence of secondary copper-mineralization in the form of malachite, azurite, and chrysocolla. Petrographic studies indicate that silicification of the pinnacle occurred prior to supergene alteration (malachite, azurite, chrysocolla, smithsonite, and hematite). Fluid inclusion temperatures of quartz from the pinnacle are bimodal. Lower temperature inclusions have homogenization temperatures from 91-110°C with salinities greater than 23 weight percent NaCl equivalent; these inclusions may be related to the typical northern Arizona breccia pipe mineralization event. Homogenization temperatures from other inclusions range from 256-317°C, having much higher values than other breccia pipe inclusions, and may be associated with nearby Tertiary volcanism. Western Nuclear, Inc. drilled 23 exploratory drill holes at the Blue Mountain pipe from 1976 to 1978. A total of 11,624 ft of rotary drilling and 1,202 ft of diamond drilling were completed within and around the pipe. Holes drilled into the pipe by Western Nuclear, Inc. and the U.S. Geological Survey (USGS) penetrated uranium-mineralized rock. The most persistent and high-grade horizon was intersected by 13 Western Nuclear holes and 3 of the 4 USGS holes. The top of this uranium-bearing zone ranges from depths of 311 ft (elevation about 5,606 ft) to ...